OFDM/OFDMA are viable modulation schemes for data transmission. OFDM has been adopted in a number of communication standards including digital subscriber line (DSL), Institute of Electrical and Electronics Engineers (IEEE) 802.11 for Wireless Fidelity (WiFi), and IEEE 802.16 for Worldwide Interoperability for Microwave Access (WiMAX). Orthogonal frequency-division multiple access (OFDMA) is a viable multiple access method based on OFDM, it enables time-frequency two-dimensional resource assignment to individual users in a communications network. OFDMA is widely used in communication standards such as IEEE 802.16 WiMAX and IEEE 802.22 for Wireless Regional Area Network (WRAN).
In most communications systems, a channel between a transmitter and a receiver introduces channel imperfections which result in errors in a transmitted message. For example, in a wireless communications system, a wireless multipath effect causes inter-symbol interference in the time domain and frequency selective fading in the frequency domain. An OFDM/OFDMA receiver can normally resolve the inter-symbol interference, but cannot recover errors caused by the frequency selective fading effect. In OFDM/OFDMA systems, messages are modulated onto subcarriers, and each subcarrier occupies a narrow-frequency band in the frequency domain. Due to the frequency selective fading in the wireless channel, signal attenuation for some frequency regions of the wireless frequency band may be significant. This is referred to as “deep fading”. Transmitted subcarriers in deep fading regions are subject to errors that make the received message encoded on affected subcarriers unreliable. Moreover, a deep fading region usually affects multiple neighboring subcarriers. Deep fading may result in a cluster of errors in the bits modulated onto deep faded subcarriers that forward error correction (FEC) coding is unable to recover due to the concentrated juxtaposition of such errors.
The classical method of solving this problem is to use FEC coding in combination with a FEC encoded bit interleaving scheme. The purpose of the interleaving scheme is to randomize the bit pattern of the FEC encoded bit sequence so that deep fading produces distributed errors in the de-interleaved received signal. This greatly improves the performance of FEC.
There are many known interleaving schemes, including one defined by the IEEE 802.22 v2.0 standard for WRAN. Some interleaving schemes interleave the FEC encoded bits as well as the subcarriers. While most known interleaving schemes significantly improve FEC performance, many are complex and computationally intensive to implement.
It would therefore be advantageous to provide an interleaving scheme that is simple to implement while achieving all of the benefits of bit and subcarrier interleaving.